An image forming method using an inkjet recording method is a method for forming an image by jetting an ink supplied from an ink tank through a channel from a recording head for ink-jetting. The inkjet recording method can form an image easily and inexpensively and therefore has been used for forming various images. As one of the inks for use in the inkjet recording method, there is an actinic radiation-curable inkjet ink. The actinic radiation-curable inkjet ink comprises a photocurable compound, and therefore an ink component can be cured by polymerizing the photocurable compound through irradiation with actinic radiation such as ultraviolet rays. When an image is formed using the actinic radiation-curable inkjet ink, it is easier to immobilize an ink jetted and an image with less bleeding can be formed on various recording media than when a solvent-based ink composition is used.
As the demand for the actinic radiation-curable inkjet ink is increasingly expanded, its characteristics are also demanded to be variously improved. Amine compounds and the like are known as an additive for improvements in ink characteristics. For example, PTL 1 discloses an UV ink containing a heterocyclic monomer and a cyclic monofunctional monomer in order that photocuring characteristics and storage stability in a thin film are enhanced without using a large amount of a photopolymerization initiator. PTL 1 also discloses an ink further containing an amine-modified oligomer as an amine compound.
In addition, PTL 2 describes a photocurable inkjet ink in which a branched oligomer is used as a curable oligomer to thereby allow an image high in hardness to be formed without any deterioration in jetting ability, and also describes use of a tertiary amine compound as a photopolymerization initiator aid. While an actinic radiation-curable inkjet ink where polyamine acrylate serving as an amine-modified oligomer is used is described in Comparative Examples in PTL 2, the ink in the Comparative Examples is low in curability.
PTL 3 describes an actinic radiation-curable inkjet ink where an oligomer having a secondary or higher amine group is used as a polymer dispersant. PTL 3 describes an actinic radiation-curable inkjet ink in which the actinic radiation-curable inkjet ink contains a polymer dispersant having a secondary or higher amine group to result in an increase in dispersion stability of a pigment, thereby suppressing surface gloss with sharpness after printing being kept.